The instant invention relates to means for sensing the acceleration profile of an object, such as a motor vehicle.
The prior art teaches magnetically-biased acceleration sensors, or accelerometers, comprising a housing having an inertial or sensing mass within a cylindrical passage therein which is magnetically biased towards a first end of the passage. Such magnetic biasing of the sensing mass offers the advantage of providing a maximum biasing force on the sensing mass when the sensing mass is in its initial position proximate the first end of the passage. When the housing is subjected to an accelerating force which exceeds this threshold magnetic bias, the sensing mass moves along the passage away from the first end thereof toward a second position at the other end thereof, with such movement being retarded by suitable damping means therefor. Where the acceleration input is of sufficient magnitude and duration to displace the sensing mass to the second position within the passage, the sensing mass triggers switch means in the sensor, as by bridging a pair of electrical contacts therein, whereupon an instrumentality connected with the switch means, such as a vehicle passenger restraint system, is actuated. In this manner, the sensor mechanically integrates the acceleration input to the housing.
An example of a magnetically-biased accelerometer is taught in U.S. Pat. No. 4,329,549, issued May 11, 1982 to Breed, wherein a magnet secured to the housing proximate the first end of the tubular passage exerts a magnetic biasing force on a magnetically-permeable ball, with the movement of the ball being damped by a gas contained within the tube. However, as the ball moves along the tube from its initial position at the first end thereof towards the contacts at the other end, the gas damping force quickly predominates in retarding the ball's movement. Thus, in the event of a loss of the damping effect due to the failure of the seal which operates to maintain the gas within the tube, any acceleration exceeding the initial magnetic biasing threshold will cause the ball to be fully displaced to the other end of the tube, thereby triggering the switch means of the sensor. In other words, an accelerometer constructed in accordance with Breed is not able to properly mechanically integrate acceleration inputs thereto in the absence of the gas damping. It is also significant that the use of gas damping requires extreme tolerance control of the gap between the walls of the tube and the ball, thereby increasing manufacturing costs.
Additionally, the ball-in-tube configuration taught by Breed may not properly integrate an acceleration input, the direction of which is not wholly coincident with the longitudinal axis of the tube: as the threshold magnetic bias is exceeded, the ball will begin to roll as it translates the length of the tube. The presence of any cross-axis vibration or transient acceleration may cause contact between the ball and other parts of the tube's inner surface such as the "roof" thereof, whereupon the ball's rotational momentum will try to direct the ball back towards the first end of the tube, even when the longitudinal component of the acceleration input is still urging the ball towards the contacts.
Still further, the magnetic bias and the gas damping featured in the Breed sensor are susceptible to unacceptable variation over temperature. Specifically, the magnetic flux generated by the fixed magnet is affected by changing temperature so as to produce significant variation in the threshold magnetic bias on the ball thereof. And, the disparate coefficients of thermal expansion of the ball and tube, as well as the changing compressibility of the damping gas over temperature, combine to adversely affect the damping characteristics of sensors constructed in accordance with the Breed patent.
Co-pending application Ser. No. 07/248,143, filed Sept. 23, 1988, now U.S. Pat. No. 4,827,091, teaches an accelerometer having a magnetic sensing mass which is magnetically biased against a magnetically-permeable element secured proximate with an end of a passage within a housing. When the housing is subjected to an acceleration sufficient to overcome the magnetic biasing force, the sensing mass is displaced towards the contacts at the other end of the passage, such displacement being damped by the magnetic interaction of the sensing mass with a plurality of electrically-conductive non-magnetic rings encompassing the passage. The contacts at the other end of the passage move longitudinally of the passage in response to temperature in order to compensate for the effects of temperature on the magnetic damping employed therein. The accelerometer further comprises a plurality of electrical coils encompassing the passage which, when energized by the delivery of direct current therethrough, effects the displacement of the sensing mass to the second position in the passage, against the contacts, whereby the operability of the sensor may be readily confirmed. Alternatively, the current is delivered through the coils in the reverse direction, whereby the magnetic biasing force is controllably increased.
Unfortunately, the accelerometer taught in U.S. Pat. No. 4,827,091, like the Breed sensor discussed hereinabove, is unable to compensate for the effects of temperature on the magnetic flux generated by the sensing mass and, hence, the sensor's threshold magnetic bias and magnetic damping force. Thus, as the magnetic flux generated by the sensing mass reversibly decreases with increasing temperature, both the threshold magnetic bias and the magnetic damping generated upon displacement of the sensing mass are correspondingly decreased, with the attendant risk that the instrumentality controlled by the sensor will be triggered by a relatively low acceleration input.
Finally, it is noted that accelerometers are frequently deployed in pairs in the interest of increased reliability, e.g., a sensor having a relatively low acceleration threshold serves to "arm" a second sensor having a relatively high acceleration threshold tailored to the particular application involved. However, in the event that the high-threshold sensor fails in the "closed" condition, i.e., incorrectly indicates an acceleration condition necessitating the deployment of the instrumentality controlled thereby, any acceleration exceeding the low acceleration threshold of the "arming" sensor will cause the deployment of that instrumentality. A graphic illustration of this condition is the deployment of an air bag upon encountering a pothole subsequent to the failure of the high-threshold sensor. It is therefore highly desirable to be able to spontaneously increase the biasing force on the sensing mass of the arming sensor and, hence, its acceleration threshold, upon an indication that the high-threshold sensor has "failed closed."